Isoflavones found in soybeans mainly include free isoflavone glycosides such as daidzin, genistin and glycitin; malonyl isoflavone glycosides such as 6″-O-malonyl daidzin, 6″-O-malonyl genistin and 6″-O-malonyl glycitin; acetyl isoflavone glycosides such as 6″-O-acetyl daidzin, 6″-O-acetyl genistin and 6″-O-acetyl glycitin; and isoflavone aglycones such as daidzein, genistein and glycitein.
Isoflavones are active constituents of soybeans which have various activities such as alleviation of osteoporosis and climacteric disturbance, and have been widely used, for example, as an additive for health foods. Accordingly, various methods for extracting and purifying isoflavones have been devised in order to efficiently ingest isoflavones.
JP-A 62-126186 (the patent document 1) discloses a method comprising extracting soybean broth or soybeans with an aqueous organic solvent under reflux to obtain an extract, bringing the extract into contact with a synthetic adsorbent resin such as a styrene-divinylbenzene polymeric resin, and then eluting isoflavones from the resin with an aqueous organic solvent (for example, ethanol or 70% methanol). In the patent document 1, although there is no detailed explanation about the concentration of an aqueous organic solvent used for extraction, 80% aqueous ethanol is used in Example 2. However, the objective of the patent document 1 is to obtain a large quantity of isoflavones at low cost, and no attention is paid to the solubility of the resulting composition in water.
JP-A 8-283283 (the patent document 2) discloses a method for purifying malonyl isoflavone glycosides comprising extracting soybeans with water to obtain an extract, bringing the extract into contact with a synthetic adsorbent resin, and then eluting the isoflavone glycosides from the resin with, for example, 20-50% aqueous ethanol. The patent document 2 states that the suitable extraction temperature is 45 to 65° C. The objective of the patent document 2 is to purify malonyl isoflavone glycosides that are relatively high hydrophilic. However, the extraction rate of the isoflavones is low because of extraction with water, and thus a composition containing a high concentration of isoflavones can not be obtained. In addition, there is no description with respect to the solubility of the resulting composition in water.
The compositions obtained by the methods described in the patent documents 1 and 2 contain a low concentration of isoflavones and do not have satisfactory solubility in water.
JP-A 2002-80474 (the patent document 3) discloses a method for obtaining a free isoflavone glycoside fraction comprising extracting soybean hypocotyls with aqueous lower alcohol (for example, 70% ethanol) at room temperature to 80° C. to obtain an extract, bringing the extract into contact with an anion exchange resin having a uniform particle diameter, washing the resin with water, and then eluting free isoflavone glycosides from the resin with, for example, 70% aqueous ethanol. The composition obtained by purification with the anion exchange resin contains isoflavones at high concentration, namely, 60% or more. However, the composition is poorly soluble in water because it is a purified product of free isoflavone glycosides having a low solubility, and thus it has a problem of instability in an aqueous solution.
Accordingly, the methods described in the patent documents 1 to 3 can not provide a composition containing a high concentration of isoflavones and having a high solubility, which can be added into beverage.
Such problem arises from the low solubility of isoflavones themselves. For example, the solubility of daidzin is only about 3.3 mg/100 ml. JP-A 10-298175 (the patent document 4) discloses a method for improving the solubility of isoflavones in water which comprises extracting soybean hypocotyls with 70 to 90% aqueous alcohol at about 70° C. under reflux to obtain an extract, bringing the extract into contact with a synthetic adsorbent resin, eluting the resin with 30 to 80% aqueous alcohol to obtain an isoflavone-containing composition, and then forming an inclusion compound of the isoflavone-containing composition with cyclodextrin. Since this method requires an addition of at least an equimolar amount of cyclodextrin to the isoflavone extract, the isoflavone content in the final product is extremely decreased. Moreover, since the solubility of the final product depends on the solubility of cyclodextrin itself, a step of dissolving the inclusion compound by heating is needed when it is used, and further, it can not be dissolved at a higher concentration.
JP-A 2000-327692 (the patent document 5) discloses a method for increasing the solubility comprising formation of α-glycosyl isoflavones by an action of glycosyltransferase in the presence of an α-glycosyl saccharide compound. However, this method also has a problem of decrease in the isoflavone content because it requires an addition of at least an equimolar amount of the saccharide compound.
Accordingly, there has been a need for development of an isoflavone-containing composition which has high purity and can be stably dissolved in water at a high concentration in a natural state without using any solubilizing agents such as cyclodextrin or any additives such as saccharide compounds and thus is suitable to use in beverage and the like.